Prion diseases or transmissible spongiform encephalopathies are infectious neurodegenerative diseases of humans and animals. A major feature of prion diseases is the refolding and aggregation of a normal host protein, prion protein (PrP), into a disease-associated protease-resistant form (PrPres) which may contribute to brain damage. In FY18, we tested three new human PrP mutations for presence of prion infectivity. In addition, we studied the role of microglia in host defense against prion disease in vivo. Human familial prion diseases are associated with mutations at 34 different prion protein (PrP) amino acid residues. However, it is unclear whether these PrP mutations can initiate generation of infectious prions. Mutant PrP itself may be neurotoxic, or alternatively, PrP mutation might predispose to spontaneous formation of infectious PrP isoforms. Using inoculation of susceptible animal models, previous reports demonstrated the presence of prion infectivity in human brain tissue expressing 7 different PrP mutations, but brains with 3 other tested mutations were not infectious. In FY18, we tested transmission using brain homogenates from patients expressing 3 untested PrP mutants: G131V, Y226X, and Q227X by intracerebral injection into tg66 transgenic mice overexpressing human PrP. Mice were followed for nearly 800 days. From 593 to 762 dpi, 4 of 8 mice injected with Y226X brain had PrPSc detectable in brain by immunostaining, immunoblot, and PrP amyloid seeding activity assayed by RT-QuIC. From these data, we concluded that the Y226X patient brain contained infections prions. From 531 to 784 dpi, 11 of 11 G131V-injected mice had possible PrPSc deposition in brain detected by immunohistochemistry, but none were positive by immunoblot or RT-QuIC assay. Due to the lack of confirmatory data, we could not make a definitive conclusion regarding the presence of prion infectivity in the G131V patient. In contrast, from 529 to 798 dpi, tg66 mice injected with Q227X brain had no evidence for PrPSc or PrP amyloid seeding activity detectable by these methods. Therefore, this brain appeared to contain no detectable prion infectivity. Transmission of prion infectivity from the patient expressing truncated human PrP, Y226X, may have implications for the spread and possible transmission of other aggregated truncated proteins in prion-like diseases such as Alzheimer's disease, Parkinson's disease and tauopathies. An early event during prion infection is the activation of microglia and astrocytes in the brain prior to damage or death of neurons. Previous prion disease studies using two different strategies to decrease microglial activity in brain reported contrary effects on survival from prion disease In the present work, in FY18, we used oral treatment with a potent inhibitor of CSF-1R, PLX5622, to eliminate 78 to 90% of microglia from mouse brain shortly after and during the course of prion infection. Oral drug treatment early after infection with the RML scrapie strain significantly accelerated vacuolation, astrogliosis and deposition of disease-associated prion protein. Furthermore, drug-treated mice had advanced clinical disease requiring euthanasia 31 days earlier than untreated control mice. Similarly, PLX5622 treatment during the preclinical phase at 80 days post-infection with RML scrapie also accelerated disease and resulted in euthanasia of mice 33 days earlier than infected controls. PLX5622 also accelerated clinical disease after infection with scrapie strains ME7 and 22L. Thus, microglia appeared to be critical in host defense during prion disease. The early accumulation of PrPSc in the absence of microglia suggested that microglia may function by clearing PrPSc resulting in longer survival.